In image capture and encoding applications, high dynamic range (HDR) video content data often is represented by a Red-Green-Blue (“RGB”) color space defined by a predetermined protocol such as ITU-R Recommendation BT.709 (also “Rec709”), ITU-R Recommendation BT.2020 (also “Rec2020”) or SMPTE's DCI P3. To transmit the HDR data over a network or to record the data on a storage device, it may be efficient to convert the HDR data to a more efficient color space such as a YUV color space. Conversion to YUV often reduces the HDR data size and facilitates compression processes which further reduce its data size. For display, however, the video content is decoded and reconverted back to RGB color space.
Conversion operations typically involve multiple stages of processing. Pixels may be represented as an RGB “triplet” of image data having a red component, a green component and a blue component. Each component of the triplet may be processed by an optical transfer function (OTF) conversion stage, which adjusts each component by a factor that represents non-linearities either in a capture device the generated the RGB data or a display device that will consume the RGB data. Following OTF conversion, the RGB data may be transformed to the YUV color space by a linear transform. The transformed YUV data may be quantized to conform each Y, U and V component to a target bitdepth. Finally, the quantized U and V components may be downsampled to match the U and V data to a subsampling protocol of the YUV data (e.g., 4:2:2, 4:2:0 or other subsampling protocol).
The conversion of the HDR image from the RGB color space to a second color space and back to the RGB color space may introduce visual artifacts to the video content. The artifacts are particularly noticeable in image regions where content is either uniform or possesses gradual change in content and they may appear as block-shaped non-uniformities that interrupt the image content. FIG. 1 illustrates an example of such non-uniformities. FIG. 1(a) illustrates exemplary source content obtained in an RGB color space. FIG. 1(b) illustrates the source content having been converted from the RBG color space, to a YUV color space and back to RGB. The conversion generates block-based discontinuities in image content that likely will be perceived as annoying artifacts by a viewer, particularly for generally uniform image content.
The inventors have identified a need in the art for an improved color space conversion protocol for video that avoid introduction of artifacts in image content.